Q51 / 180 · COMEDK 2025 (10 May Shift 1) · Chemistry
Choose the correct statement.
- AIonic compounds of $\mathrm{Sc}^{3+}$ and $\mathrm{Cu}^{+}$are coloured because of $\mathrm{d}-\mathrm{d}$ electronic transitions
- BThe order in which the paramagnetic nature of the 4 cations $\mathrm{Cr}^{2+}, \mathrm{Mn}^{2+}, \mathrm{V}^{2+}$ and $\mathrm{Fe}^{2+}$ vary is $\mathrm{V}^{2+} <\mathrm{Cr}^{2+}=\mathrm{Mn}^{2+} <\mathrm{Fe}^{2+}$
- CThe metal Cobalt has the electronic configuration $[\mathrm{Ar}] 3 \mathrm{~d}^5$ in the +3 oxidation state
- Correct answerDAs the oxidation number of the transition element increases, the ionic nature decreases and the oxides show acidic nature predominantly
Solution
(1) Incorrect: $Sc^{3+}$ has $[Ar]3d^0$ and $Cu^+$ has $[Ar]3d^{10}$. With completely empty or completely filled $d$-orbitals, $d$-$d$ transitions are not possible. Their compounds are colourless.
(2) Incorrect: Paramagnetism depends on number of unpaired electrons $n$.
$V^{2+}(3d^3) \to n=3$, $Cr^{2+}(3d^4) \to n=4$, $Mn^{2+}(3d^5) \to n=5$, $Fe^{2+}(3d^6) \to n=4$
Correct order: $V^{2+} < Cr^{2+} = Fe^{2+} < Mn^{2+}$
(3) Incorrect: Co is $[Ar]3d^74s^2$. In $+3$ state ($Co^{3+}$), it loses $2$ electrons from $4s$ and $1$ from $3d$, giving $[Ar]3d^6$, not $3d^5$.
(4) Correct: By Fajans' rules, higher oxidation state increases polarizing power, increasing covalent character and decreasing ionic nature. Higher covalent character in oxides leads to predominantly acidic nature (e.g., $CrO_3$, $Mn_2O_7$ are acidic while $MnO$, $CrO$ are basic).
The correct statement is (4).
(2) Incorrect: Paramagnetism depends on number of unpaired electrons $n$.
$V^{2+}(3d^3) \to n=3$, $Cr^{2+}(3d^4) \to n=4$, $Mn^{2+}(3d^5) \to n=5$, $Fe^{2+}(3d^6) \to n=4$
Correct order: $V^{2+} < Cr^{2+} = Fe^{2+} < Mn^{2+}$
(3) Incorrect: Co is $[Ar]3d^74s^2$. In $+3$ state ($Co^{3+}$), it loses $2$ electrons from $4s$ and $1$ from $3d$, giving $[Ar]3d^6$, not $3d^5$.
(4) Correct: By Fajans' rules, higher oxidation state increases polarizing power, increasing covalent character and decreasing ionic nature. Higher covalent character in oxides leads to predominantly acidic nature (e.g., $CrO_3$, $Mn_2O_7$ are acidic while $MnO$, $CrO$ are basic).
The correct statement is (4).
Hence, the correct answer is option: 'D'.